Line with negative resistance loading



Oct. 24, 1944. A R 236,1)

LINE WITH NEGATIVE RESISTANCE LOADING Filed Jan. 16, 1945 EL/firHmM/srok 172/6 l5 4 LOADING POTENTIAL IIV VOLTS //v Vf/VTOR L. G.ABRAHAM Patented Oct. 24, 1944 LINE WITH NEGATIVE RESISTANCE LOADINGLeonard G. Abraham, Madison, N. J., .assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication January 16, 1943, Serial No. 472,588

Claims.

The present invention relates to the supply of power over a line tonegative resistance devices associated with the line.

Negative resistances have been proposed for loading transmission linesto reduce the line attenuation for signal or other waves transmittedover the line. The negative resistance units operate over a droopingportion of their volt-ampere characteristic and it is necessary tosupply them with energizing or bias current to bring them into theportion of the characteristic that has negative slope to enable them todevelop a negative resistance. It is generally desirable to supply thisbias current from a terminal point over the line conductor or conductorsto a number of the loading units along the line. I

In installations of this type a difiiculty arises in starting the systeminto operation in that initially a relatively high voltage must beapplied to the line to bring the negative resistance devices into theregion of their characteristic that has a downward slope, after whichthe applied voltage must be reduced to hold the devices at the properoperating point on their characteristic and, as well, to avoid danger ofharmful effects to the devices resulting from excessive current flowthrough them.

An object of this invention is to provide means for automaticallyinsuring application to such a system of the proper starting voltage forinitially bringing the devices into-their intended operating range andfor thereafter reducing the applied voltage to the proper operatingvalue.

The nature of the invention and its various features and objects will bemore readily apparent from the following detailed description ofillustrative embodiments thereof, show-n in the accompanying drawing.

Referring to the drawing:

Fig. 1 is a diagrammatic showing of a telephone line with negativeresistance loading using thermistors as the negative resistance'devicesand embodying the invention inone form;

Fig. 2 is a diagram showing the volt-ampere characteristic of one of theloading devices of Fig. 1; and

Figs. 3 and 4 show other forms of embodiment of the invention intransmission circuits schematically shown.

Referring first to Fig. 1, the line In is shown connected (through themedium of exchanges with suitable switches, not shown) for two-waytransmission between subscriber stations H and 12. The line is provided.with negative resistance loading, the negative resistance loading unitsso that very little current flows.

being shown spaced along the line conductors at I3, l3, etc. with aspacing Z. Energizing current for the negative resistance loading unitsis supplied over the line conductors from a source M in series with anohmic resistance 15 with a ground return irom H3 at the opposite end ofthe line. Repeating coils are shown at I! and .18 at opposite ends ofthe .line, with terminal loss pads l9 and 20. The negative ressitancedevices may comprise vacuum tube circuits of suitable type fordeveloping series type negative resistance or any other suitable typesof devices capable of developing requisite negative resistance effectsin the line, one specific type of device being a thermistor, suchloading being more fully disclosed in an application of R. K.Bullington,Seria1 No. 440,549, filed April 25, 1942, to which referenceis made for further detailed disclosure. The pads l9 and 20 are used toprovide a suitable over-all loss which may be desirable for crosstalksuppression or other transmission control and their use also aids inpreventing terminations using different types of subscribers lines fromcausing the circuit to sing. The loaded line I'll may have .a zero loss,negative loss or small loss per mile following the teachings of theBullington disclosure.

Negative resistance devices have a volt-ampere characteristic of thegeneral form given in Fig. 2,

the curve as plotted in this figure being a typical characteristic of athermistor such as would be suitable for the loading of telephone lines.As voltage increasing from zero is applied across such a thermistor(through a series resistance) the thermistor resistance is firstpos'itve and high When the applied voltage reaches '30 volts the currentis 3 milliamperes. The characteristic bends over at about 30 volts, theslope becoming negative with increasing current, with consequentdecrease in voltage drop as the current increases. At the 10-milliamperepoint (b) the voltage drop has fallen to 14 volts. Since this point.lies about midway of a considerable range :of nearly constant slopebetween a and c, the :point rb is a suitable operating point and thebias voltage for operation about this point is, therefore, 14 volts. Forpurposes of illustration it will be assumed that each of the negativeresistance .devices l3 in the line 1'0 has a characteristic like that ofFig. 2, although it will be understood that widely difierent valuesmight apply to different cases in practice. For further purposes ofillustration, let it be assumed that the line in Fig. .1 comprises ten"negative resistance loads in each conductor with ten loading sectionseach having a line conductor resistance of 400 ohms, giving a totalresistance of 4,000 ohms for each line conductor. The sum of thevoltages at the loads required to start the ten thermistors is 10 30 or300 volts while the voltage required to bias them to their working pointis 10x14 or 140 volts. It is desirable with the type of regulationdisclosed in Fig. 1 to make the voltage of battery 4 and resistancevalue of l5 such as to fulfill these two conditions. Calling R, thevalue of resistance l5, and noting that the line current is .003 amperewhen the starting voltage is 30 volts per load and is .010 ampere forthe working condition, the voltage supplied by the battery in bothconditions is E=.003 R+.003 (4000)+ 30 =.01 R+.01 (4000) +10 140 Solvingfor R,

R=18,857 ohms Hence E=368.5'7 volts That is, a battery voltage at I4 ofthis voltage and a resistance at l5 of this resistance will just allowthe required 30 volts per load to be developed for starting and willlimit the steady current to .010 ampere and the steady voltage to 14volts per load. For reliability of operation somewhat larger values ofboth E and R may be desirable, such that as the current increases, thedrop through R. increases somewhat faster than the rate of fall ofpotential in the negative resistance devices. For example, if thebattery voltage is 380 and resistance R has a value of 20,000 ohms, thevoltage put across each load at a current of .003 ampere is 30.8 voltswhile at .010 ampere it is 14 volts. In the working condition there is avoltage drop in R of 200 and a drop of 40 volts in the line resistanceleaving 380-240=140 volts to be applied to the loads. The foregoingcalculation shows that if the line resistance were greater than theassumed value, R could be correspondingly smaller. In fact, theresistance l5 could be entirely absorbed in the line by using asufficiently high line resistance.

Stability is another consideration in choosing the size of resistance R.A small increase in battery voltage tends to increase the line currentand the increase in current through the positive resistances of thesystem produces a drop of potential in a direction toabsorb the increasein battery voltage; but the effect of the assumed increase in current inthe negative resistance devices is to give back a Voltage rather thanabsorb voltage so that if the total amount given back in this way isgreater than the amount absorbed,

the net efiect of the initial assumed increase in battery voltage is afurther increase in the same direction. This is indicative of anunstable condition. It can be prevented by increasing the value of thepositive resistance in the circuit. As an example, assuming the samecircuit constants as given above, suppose that an increase of currentfrom .010 to .011 ampere results in a decrease of terminal voltageacross each thermistor of .5 volt. Then the decrease for ten thermistorsis 5 volts. In order to provide an increase in voltage drop of 5 voltsresulting from an increase in current amounting to .001 ampere, R musthave such value that or R=1000 ohms. So long as R is in excess of 1,000ohms, therefore, the system will be stable as regards fluctuations inthe power supply. Obviously, in this case if R. has a value such assuggested previously for supplying the proper starting and bias voltagesto the thermistors, the stability requirement is also met since thevalue of R is greatly in excess of the 1,000 ohms found from the assumedconditions.

Fig. 3 shows .a type of loaded line circuit which permits the use of alower voltage battery than in the case of the Fig. 1 type and whichconsumes less power. The system of Fig. 3 differs from the system ofFig. 1 in having relays 25, 26, 21, 28, etc. connected in circuit atsuitable points along the line and in having resistors 30, 3|, 32, 33,etc. connected to the line wires under control of these relays in amanner and for a purpose to be disclosed.

Assuming the same circuit constants for the line and loads as before,there would be a variety of possible values for the battery voltage andsizes of the resistances I5, and 30 to 33, but it is thought that theprinciple involved can best be illustrated by arbitrarily assuming a setof values and noting their effect. Suppose, therefore, that the voltageof battery M be taken as 250 volts and the values of resistances I5, 30and 32 be taken at 3,500 ohms, 20,000 ohms and 10,000 ohms,respectively, resistance of 3| being equal to that of 30, and resistanceof 33 being equal to that of 32. Confining attention at first to theleft-hand portion of the circuit from the battery up to the point atwhich the first pairof resistors 30, 3| is located, in the startingcondition in which the current must reach at least the value .003 amperebefore the resistance of the thermistors begins to decrease, the drop ofpotential in 15 is 2 .003 3,500=21 volts, leaving an applied linevoltage of 250-21=229 volts. It will be assumed that the resistances 30and 3| are located half way between the fifth and sixth thermistorscounting from the left along the line. If the total resistance of eitherside of the line beyond resistances 30 (or 3|) is 20,000 ohms, this inparallel with 30 (or 3|) gives a resultant of 10,000

; ohms through which the intial .003 ampere must per thermistor, this isample under the assump- .008 2 3,500 drop through R +.008 5 400 dropthrough five sections ofline +.008 10,000 drop through total resistanceto right of point of connection of 30, 3|

+19.6 5 drop across five thermistors giving a total of 250 volts. Inthis way the first five sections are brought into operating conditionwith the application of no higher voltage than the assumed 250 volts.

Thermistors 6 and I are brought into operating condition by the dropexisting across 30 (or 3i) when the line current has reached the assumedvalue of .008 ampere as assumed. Resistors 32, 33, as stated, each ha avalue of 10,000 ohms and if the total resistance between the point ofconnection of each to the line and the ground at It is 10,000 ohms, theresistance of the parallel combination is 5,000 ohms, each side. Supposethat of the .0008 ampere flowing over the first five sections of line,at least as much as .005 is flowing through either resistor 30 or 3!.The other .003 ampere Will then take the path through line sections 6and l. The line drop of 400 .003=1.2 volts per section will consurne 2.4volts and the assumed 5,000 ohms total, to the right of the point ofconnection of 02 or 30, will consume .003 5,000=15 volts making a totalresistance drop of 17.4 volts which, subtracted from the assumedavailable applied 100 volts from across resistor 30 or 3|, leaves 82.6volts or 41.3 volts per thermistor for starting thermistors l and 8.This is well above the required 30 volts per thermistor. Thermistors andl will, therefore, be started.

Relays 25, 20 will each operate to attract its armature in response to aline current of about .008 ampere so that resistors 30 and 3! areremoved from circuit when that value of line current is reached. Thecircuit for the first seven sections will now stabilize at, say, .0085ampere and 17.74 volt drop across each thermistor. [.0085 (2X3,500+7400+5,000)+7 l'7.'74=250 volts] This current divides between resistor 32or 33 and the total circuit resistance beyond that point. If .003 flowsthrough the latter branch, the drop acros 32 (or 03) will be about 55volts which is sufficient to start thermistor 0. Relays 2i and 20operate on a line current of about .008 ampere and remove resistors 32and 33 from circuit. In similar manner to that described thermistorsection 9 and iii are started, two further, pairs of relays beingrequired following relays 21 and 28 in order to allow sections 0 and iiito be started and the last of the starting shunts to be removed. Thisexample shows, therefore, how a ten-section line can be started with anavailable voltage of less than ten times the starting voltage of onethermistor.

Fig. 4 shows a similar starting circuit applied to a circuit comprisingthe two line conductors in series. Battery sections 00, 42 are connectedat opposite ends of the line and are poled to be series aiding. They areconnected through stabilizing resistances i l, 43 which as in theprevious cases could be absorbed in the line as conductor resistance ifdesired. This voltage is applied across condensers inserted in themiddle of the repeater coils ll, 58. In this case a single shunt 46 canbe used at the first shunting point, connected across the line andarranged to be opened when the lin circuit through the winding of relay45 reaches sufficient strength. The talking currents are by-passedaround the relay windings by condensers ll, 48. Other shunting pointsmay be provided as necessary, one other shunting relay 50 beingindicated, controlling shunt resistor 40. The operation of this circuitis similar in general to that described of Fig. 3.

The invention is not to be construed as limited to the specific circuitdetails or quantitative values that have been given since these ar forpurposes of illustration in order to provide a clearer understanding ofhow the invention may be applied to particular cases. The scope of theinvention is defined in the claims.

What is claimed is:

1. In a transmission system, a line, negative resistance loading unitsconnected at intervals along said line, said units each requiring avalue of operating direct current bias voltage that is low relative tothe starting voltage required to bring them into the negative resistanceregion of their characteristic, a source of direct current bias voltageat a point on said line for supplying bias current over said line to allof said units and means to apply to said line initially a sulficientlarge proportion of the voltage of said source to initiate the negativeresistance condition in said units, said means being effective inresistance loading units connected at intervals along said line, saidunits requiring a value of operating direct current bias voltage that islow relative to the starting voltage required to bring them into thenegative resistance region of their characteristic, a source of directcurrent bias voltage at a point on said line for supplying bias currentover said line to said units, one or more shunt resistances initiallyconnected to one or more points in said line between certain of saidunits remote from said source to provide relatively low returnresistance for the bias current whereby a greater bias voltage isapplied across certain of said units than across others for startingpurposes, and means for removing said shunt resistances in response tocurrent flow of given strength over said line.

3. In a transmission system, a line, negative resistance loading unitsinserted therein at periodic intervals, a source of bias current commonto a plurality of said units, a path over said line in series with saidunits and said source for the flow of said bias current, a shuntingresistance initially closed across said path at a point between certainof said units to permit application initially or" a large enough voltageto the units included between said source and said shunting point tobring the latter units into their negative resistance condition wherebyan increased bias current flows through them, and means operating inresponse to said increased current flow to remove said shuntingresistance from across said path.

4. In combination, in a signaling system, a transmission line,thermistors included therein at periodic intervals, a source of biascurrent for biasing all of said thermistors to the negative resistanceregion of their volt-ampere characteristic whereby said thermistorsintroduce negative attenuation into said line for signal currentstraversing said line, a path for said bias current extending from saidsource over said line and through said thermistors in series, shuntresistors initially connected across said path at spaced points alongsaid line between different thermistors whereby initially thethermistors nearest said source receive the greatest bias voltage, thebias voltage so received being sufficient to bring the latterthermistors into their negative resistance condition thereby increasingthe flow of bias current through the first shunt resistance suflicientlyto bring a thermistor located beyond said first shunt resistor into itsnegative resistance condition, and means to remove each of said shuntresistors in succession in response to increased bias current flow overthe preceding line section.

5. In a thermistor loaded telephone line having thermistors inserted inthe line at loading intervals to introduce negative resistance in theline for speech currents, the method of biasing all of said thermistorsinto their negative resistance region without having to provideexcessively high biasing voltage in the bias current source, comprisinginitially shunting the bias current path over the line at one or morespaced points along the line to permit the thermistors located in asection of line nearest the bias current source to receive high enoughvoltage to bias them into their negative resistance region resulting inan increased flow of bias current, using said increased flow of biascurrent to apply high enough voltage in the next succeeding line sectionto bias the thermistors located therein to their negative resistancecondition, and removing one at a time said one or more shunting pathsfrom the bias current path until all thermistors have been brought intotheir negative resistance condition and all initial shunting paths havebeen removed.

' LEONARD G. ABRAHAM.

